The roman emperor constantine legalized christianity with the edict of milan in 313 ce. he built new churches using a plan: a rectangular structure with a long central nave (or main hallway), two or more side aisles running alongside the nave, a transept that cuts across the nave to make a cross shape, and an apse (semi-circular niche or space) at the back of the church behind the altar.

A rectangular structure with a long central nave

Explanation:

Basilicas originally referred to the ancient Roman public buildings. Courts were held in these buildings and served other public functions. It had door at the one end and a raised platform on the other side, the magistrates and other officials used to sit on the raised platforms. The basilicas were located in the centre of all Roman towns.  

Later when Christianity became the official religion of the Roman Empire churches were built on the plans similar to the basilicas. Basilicas had open interior space and its side isles were separated by colonnades.

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answer:

it caused mass african migration, african populations to grow, african empires to topple, and racism against slaves to emerge.